One important type of a rotary drill bit for rock drilling for oil wells and the like uses rolling cone cutters mounted on the body of the drill bit so as to rotate as the drill bit is rotated. Such a rock bit has a sturdy steel body which is threaded onto the lower end of a drill string and rotated in the hole being drilled. A number of cones, commonly three, are mounted on the rock bit body for engaging the bottom of the hole being drilled. Each of the cones is mounted on a bearing pin aligned so that as the drill bit is rotated each of the cones rotates about its own axis. High performance rock bits often include tungsten carbide inserts pressed into insert holes in the external surface of the cutter cones. These tungsten carbide inserts bear against the rock formation at the bottom of the hole, crushing and chipping the rock as drilling proceeds.
Such rock drilling is very demanding service and construction of the rock bit must be quite rugged. The cones on such a rock bit are heat treated to substantial hardness and carefully prepared bearing surfaces are needed to avoid premature bearing failure during service. Close quality control of the cones, as well as other elements of the rock bit, is essential.
Prior manufacture of tungsten carbide insert cones for rock bits has commenced with forged steel bodies of generally conical shape. Such a body is machined to form a generally cylindrical bearing cavity substantially coaxial with the conical external surface. A variety of bearing and sealing surfaces can be provided in the bearing cavity. One such surface comprises a circumferentially extending ball bearing race in the generally cylindrical cavity. It has been found desirable to have a hardness at the surface of the ball bearing race greater than the hardness of the core of the cone.
After machining the bearing cavity and, in some embodiments, portions of the external surface of the cone, the ball bearing race has been selectively carburized for enhancing hardness. The carburizing grades of steel have low carbon content, i.e., less than about 0.25% carbon. Typically low carbon steels for rock bit cones have no more than about 0.15% carbon. This assures a substantial difference in carbon content between the core and the carburized case, resulting in a relatively tough and ductile core and a high hardness case at the surface. If a higher carbon content steel were used, the heat treating cycle needed to harden the carburized case would result in excessive hardness in the core of the cone. Typical steels for forming tungsten carbide insert cones for a rock bit are types 9310, 9315, 4815, or 4820.
Carburizing of the ball bearing race involves "stopping off" areas on the cone where carburizing is not desired, such as on critical bearing surfaces. Two layers of a refractory coating are hand painted onto the surfaces of the cone where carburizing is to be inhibited. Such coatings must be carefully applied to avoid pinholes which would lead to carbon "leakage" and unwanted hard spots on the surfaces. The cone is then placed in a carburizing pack or atmosphere and held at elevated temperature for a sufficient time to produce a carburized layer on surfaces exposed through the stop-off material. A case depth of as much as 0.065 inch may be formed in order to provide excess material for subsequent machining operations. A case with a carbon content as high as 0.90% can be produced on a cone with a core carbon content of only about 0.15%. After carburizing, the cone is slowly cooled then annealed to be in suitable condition for machining.
After carburizing and annealing, the exterior of the cone is machined to its final profile. This removes the carburized case from areas where holes are later to be drilled. In some instances the cone is rough machined before carburizing and the lands where holes are to be drilled are finish machined after carburizing and annealing. This increases the number of machining set-ups. The cone is then heat treated by oil quenching from the austenitizing temperature and tempering at about 400.degree. to 500.degree. F. Such low temperature tempering makes little, if any, change in the as-quenched core hardness of the cone and largely relieves stress maldistribution and increases toughness, which help prevent cracking. Insert holes are drilled in the external surface of the cone for insertion of tungsten carbide inserts. Tungsten carbide inserts are press fitted in place in such holes. Various bearing surfaces in the internal bearing cavity, including the ball bearing race, are ground to final dimensions either before or after final heat treating, or before or after press fitting of the inserts.
Heat treating seeks to achieve a yield strength in the core of about 150,000 psi or a hardness of about Rockwell C-42 and a case hardness in the ball race of about C-55 to C-60. If the strength of the core is too low the press fitted tungsten carbide inserts may be loosened during use of the rock bit. Unwanted rotation or even loss of inserts can occur. If the strength and hardness of the core are too high, with consequent low ductility, breakage of a cone can occur leading to severe problems during well drilling. If the carburized case in the ball bearing race is too soft, surface damage and wear can occur as the rock bit is operated, leading to premature bearing failure. Excessive hardness in the ball race can initiate cracking which, if propagated through the wall of the cone, can result in cone breakage. Since the entire cone is heat treated after carburizing, the hardness of the carburized case is dependent at least in part on the heat treating cycle needed to obtain the desired strength in the core. Further, the higher carbon content in the carburized case can lower hardenability of the case in some steels. Complete solution of alloys and carbon in austenite may not be obtained in subsequent heat treating operations, with an adverse effect on hardness distribution in the finished cone.
Holding close tolerances on the strength and hardness of both the case and the core in a carburizing grade steel with low carbon content is quite difficult. Tempering a hardened steel core to a desired final hardness is not practical because of adverse loss of control of the hardness of the carburized case. The as-quenched core hardness in a rock bit cone is sensitive to composition of the steel, and such steel is often purchased at premium prices with composition tolerances smaller than usual steel industry standards.
In addition to the premium cost of materials and difficulty in maintaining proper core strength, the requirement for carburization of the ball race imposes substantial cost. This includes capital cost of equipment with sufficient capacity for carburizing and slow cooling the entire production volume of cones, as well as the labor of hand painting the cones to prevent unwanted carburizing. Because of the numerous steps needed for carburizing, there can be substantial work in progress in the manufacturing facility. Inadvertent carburization due to leakage through the stop-off materials can lead to hard spots on bearing surfaces and on the external surface of the cone, which can interfere with subsequent machining. Close quality control of sensitive manufacturing operations can lead to costly scrapping, reworking or diversion of a portion of the products.
It is therefore, desirable to relax the specifications for composition of steel, simplify manufacturing operations, ease quality control problems, and improve or at least not degrade the quality of tungsten carbide insert cones for rock bits.